The present invention relates to a multilayer-coated cutting tool used for cutting metallic materials, etc., particularly to a multilayer-coated cutting tool with improved wear resistance and oxidation resistance.
Due to an increasing demand for a higher efficiency of cutting, high-speed machining centers have come into wide use, resulting in a trend of a higher cutting speed. According to this trend, it has become common to coat cutting tools with TiAlN having an improved oxidation resistance in place of TiN and TiCN.
To cope with further increase in the speed of the cutting operations, various improvements are proposed on the coatings of cutting tools. For instance, Japanese Patent 2,793,773 proposes the addition of Si to a TiAlN coating film to improve its oxidation resistance, and Japanese Unexamined Patent Publication Nos. 8-118106 and 9-11004 propose the addition of Si to a Ti-based coating film.
However, the mere addition of Si to the conventional coating film of TiAlN can improve oxidation resistance at most less than 1.2 times, failing to meet the present demand of higher cutting speed, though it produces some effects for a general cutting purpose. Further, though the addition of Si to a Ti-based hard coating film can slightly improve its oxidation resistance, it cannot sufficiently improve a static wear resistance of the coating film, resulting in a coated cutting tool not showing a sufficient improvement. This appears to be caused by the fact that Si added to the coating film forms a simple hard solid solution phase in which Si atoms substitute for Ti atoms to provide only solid-solution strengthening.
Further, the coating film simply containing Si is more brittle by remarkably large compressive stress than a coating film containing no Si, and this excessive compressive stress makes the coating film prone to peel off from the cutting tool substrate immediately after the coating film is formed. Accordingly, the Si-containing coating films have not practically been applied to the cutting tools so far. As works become harder and the cutting conditions become severer, abnormal wear and fracture are caused by the peeling and oxidation of the coating film, resulting in failure to use cutting tools with Si-containing coating films for practical applications. Thus, sufficient improvement has not been achieved so far in the coating films of cutting tools to provide cutting performance necessary for high-speed cutting operations.
Accordingly, an object of the present invention is to provide a cutting tool with an Si-containing coating film having drastically improved wear resistance and oxidation resistance without sacrificing adhesion to the cutting tool substrate, thereby fully exhibiting its own characteristics so that the cutting tool is highly suitable for high-speed cutting operations.
In view of the fact that an Si-containing coating film formed on a cutting tool substrate has extremely higher compressive stress than a coating film not containing Si, and that the Si-containing coating film sometimes peels off during cutting operations due to an excessive compressive stress, failing to apply the Si-containing coating film to the cutting tool, the Si-containing coating film is desirably used in combination with another hard coating film having excellent adhesion to the substrate. By controlling the structure and crystal form of the Si-containing coating film itself, the residual compressive stress of the Si-containing coating film can be reduced, further improving the adhesion thereof to the cutting tool substrate. Further, by controlling the crystal form of the Si-containing coating film, the hardness of the coating film can be extremely increased, thereby further improving the wear resistance of the coated cutting tool. It has also been found that the oxidation resistance of the Si-containing coating film can be remarkably improved by controlling the crystal form thereof.
Most of presently available multi-element nitrides such as TiAlN or the like form nitrides having a cubic NaCl-type crystal structure, and when Si is added to TiAlN, Si substitutes for a Ti atom in TiAlN to generate a lattice strain owing to a difference in an atom radius between Si and Ti, resulting in increase in a residual compressive stress. As the amount of Si added increases, the compressive stress excessively increases. It has thus been impossible to add Si to multi-element nitrides such as TiAlN, to such an extent that Si can provide sufficient effects.
As a result of research in view of the above problems, the inventors have found that the Si-containing coating film comprising Si and one or more metallic elements of Groups 4a, 5a and 6a and Al can be provided with reduced stress and improved wear resistance as well as improved adhesion to the cutting tool substrate, by making the Si-containing coating film have a structure in which Si-rich, hard crystal grains are dispersed in a matrix constituted by a phase containing a relatively small amount of Si and having a small compressive stress.
Thus, the multilayer-coated cutting tool according to the present invention comprises a cutting tool substrate, and a multilayer coating film comprising a first hard coating film formed on the substrate and a second hard coating film formed on the first hard coating film, the first hard coating film comprising one or more metallic elements selected from the group consisting of Ti, Al and Cr, and one or more non-metallic elements selected from the group consisting of N, B, C and O; and the second hard coating film comprising Si and one or more metallic elements selected from the group consisting of metallic elements of Groups 4a, 5a and 6a of the Periodic Table and Al, and one or more non-metallic elements selected from the group consisting of N, B, C and O; the second hard coating film being a composition-segregated polycrystalline film comprising a phase having a relatively high Si concentration and a phase having a relatively low Si concentration.
The second hard coating film preferably has an average grain size of not more than 50 nm. In the second hard coating film, the phase having a relatively high Si concentration constitutes an amorphous or microcrystalline phase. The second hard coating film preferably comprises Si3N4 and/or Si as a single phase.
In one embodiment, the second hard coating film preferably contains a combination of Cr and Si or a combination of Ti and Si as metallic elements.
In a further embodiment, the second hard coating film contains boron in the form of a boron nitride. In a still further embodiment, the second hard coating film is a multilayer coating film comprising at least two or more of a CrSiN layer, a (CrSi)2N layer, a CrSiBN layer, and a (CrSi)2BN layer.
In a still further embodiment, the first hard coating film preferably contains a combination of Ti and Al or a combination of Cr and Al as metallic elements.
In a still further embodiment, the first hard coating film comprises Al partially replaced by at least one element selected from the group consisting of Si, Mg, Ca, Sr, Li, K and Y in a range of 0.5 atomic % to 30 atomic %.
In a still further embodiment, both the first and second hard coating films are formed by an arc discharge ion-plating method. Both the first and second hard coating films inevitably contain a plurality of droplet particles.
The cutting tool substrate may be an end mill or an insert of cemented carbide.